Marketing departments often find it useful to have products available at virtually every price point, so may create products with mere finish variations in order to give the higher-paying customer a feeling of getting more for their extra money.

RX100 was touted as a racing gruppo, but got almost no advertising money devoted to it. And perhaps for the same reason, RX100 was practically an OEM-only gruppo.

It was a great gruppo imo, I built my first STI-equipped bike about 25 years ago using a pair of RX10 levers I bought new at a swap meet, and I still have the late-60's PX10 with those levers on it!

For mechanical differences between RX100 and same-period 105, look at the hub bearing shielding. I think that 105 may have used plastic pieces and a more effective seals there.

Oh, and here's that PX10 that I pulled from The Off Ramp's dumpster in 1996 and rebuilt using RX100 levers, Roval wheels, Sachs rear derailer and Super68 calipers: